One of the most critical limiting factors affecting spacecraft operations is the availability of sufficient electrical power for those operations. Virtually all spacecraft operations demand electrical power. Examples include imaging, transmitting and receiving communications signals, sensor operation, data processing, spacecraft attitude adjustment and other activities related to the spacecraft's mission or the maintenance of its trajectory or orbit. The principal (if not sole) renewable source of energy for such operations is solar power, which is typically captured by one or more photo-voltaic solar panels attached to the spacecraft exterior.
Solar panels are typically flat or planar with sensitivity on one face. However, they may otherwise take on a variety of shapes and sizes. Their disposition with respect to the spacecraft and each other will likewise, vary, depending on constraints imposed by spacecraft design and mission objectives. The amount of power captured by a flat solar panel varies directly with the extent to which it faces the sun (i.e., the extent to which the plane of the flat surface is perpendicular to the direction of the sun). When a flat solar panel is perpendicular to the direction of the sun with the sensitive face oriented to the sun, the maximum exposure is obtained, and when oriented 90 degrees or greater from perpendicular, the minimum exposure is obtained. Using the optimal or maximum solar exposure as a standard, a critical question in spacecraft operations is the percentage optimal exposure (hereinafter the "exposure ratio") occurring at various times during a given time interval. This information is required for missions such as an interplanetary voyage or an orbit around the earth or another body. The exposure ratio will determine, for example, whether a given set of operations can be performed during a given portion of the satellite orbit around the earth. If the power requirement is greater than the power available, it may be necessary for the satellite to shut itself down or delay operations during certain periods.
For the reasons stated, information on the exposure ratio is critical to mission planning. With respect to earth-orbiting satellites, exposure ratio is a major factor to be considered in determining the possible orbits (inclination, altitude, eccentricity, precession, etc.). This is critical as the orbit determines where the satellite will be at all times. The exposure ratio also plays a role in spacecraft design, including features of the spacecraft's shape and the size and position of objects (such as telescopes or sensors) mounted on its exterior, as well as the size, shape and placement of the solar panels themselves.
Traditionally, calculation of the exposure ratio is accomplished by applying complex and time-consuming mathematical operations to extensive data on the spacecraft itself, the orbit, the spacecraft attitude and other factors. Moreover, while this process may yield numerical data relating the exposure ratio to time, it does not provide an easy and reliable way to visually model the data, thereby facilitating alternative spacecraft designs or orbits to maximize the exposure ratio.
In a problem related to exposure, the drag acting on a satellite will determine how much fuel must be used to compensate for the drag, and the drag will also determine the orbit degradation over time. In a manner similar to that used to calculate exposure, traditional approaches have been heavily dependant on time-consuming calculations with very complex models. Further, existing orbital drag models are not highly accurate and mission planners rely heavily on historic orbit degradation to predict future degradation. This shortfall in drag models makes it difficult for satellite designers to optimize the satellite design and minimize drag effects.
To overcome the shortcomings of the traditional approach, a method and apparatus are needed that will quickly and reliably model the time-varying spacecraft exposure ratio and drag and, at the same time, allow for visual data display thereby facilitating alternative spacecraft design to optimize solar exposure and minimize drag.